Support OPML for feed

Hi there,

It will be nice to have an ability to use OPML file for feeds, like `!hookshot opml ` to import multiple feeds at once

Start by locating the command handling for `!hookshot` and the existing feed support, then trace how a feed URL is fetched and registered. Define the OPML URL input and determine how multiple feeds should be imported; the work is done when one command successfully imports all feeds described by a valid OPML file.

Written by the indexing model from the issue text.